1. Field of the invention:
The invention concerns gas turbine stages which entail devices for cooling the periphery of the turbine wheel disc by means of blowing in air in the direction of the upstream side of the disc. It relates in particular to the turbines of aviation turbojets.
2. Description of the Prior Art:
In the turbine stages of the type referred to above, there are two flows of gases which must remain distinct from each other insofar as possible. The first is the circular flow of the combustion gases which drives the blades of the wheel. The other is the flow of air which circulates around the disc and whose role is simultaneously to cool the disc and to balance the pressures between the jet and the upstream and downstream enclosures delimited by the wheel. These gas and air flows are highly centrifuged because of the high speed of operation of the turbine. This phenomenon has one favorable consequence, namely preventing the migration of hot gases toward the disc, and one unfavorable consequence, that of impeding the circulation of the combustion gases by constricting the jet.
It is known that the blades are often provided with platforms. More specifically, the foot of a blade consists of three parts: the blade root which fits into a housing in the periphery of the disc, the shank on top of the root, and finally the platform atop the shank which supports the airfoil portion. The presence of the platform increases the inertia of the blade. It thus lowers the blade's own resonance frequencies and consequently the critical velocity or velocities of the turbine. In addition, when all the platforms of a wheel are interconnected, they make it possible to form a crown which promotes the separation of two flows and, with the periphery of the disc and the shanks, delimit passages which channel the cooling air around those parts of the disc which are most subject to heating. Obviously, this function is provided only when the platforms are interconnected. Even in this case, a significant fraction of the air flow is deflected by the upstream side of the disc and by the upstream surfaces of the blade shanks. This results in a reduction of the yield of the stage because of the turbulent reintroduction of centrifuged cooling air into the flow of combustion gases, as well as in a lack of cooling efficiency.
Another known practice, shown for example in French Pat. No. 1,417,600, is the use of elastic trough-shaped dampers whose function is to dampen the vibrations of the blades and to further lower the frequency of those vibrations. Each of these dampers is inserted in the passage delimited by the shanks of two adjacent blades, the platforms thereof, and the periphery of the disc. The damper takes up one portion of the cross section of the passage and is supported on its edges by the periphery of the disc while its back pushes against the platforms. In operation, each damper is subject to centrifugal force and lowers the resonance frequencies of the neighboring blades by the pull it exerts on the shanks through the intermediary of the platforms. The vibrations are dampened by the friction of the dampers in contact with the platforms.
The purpose of the invention is to take advantage of the presence of such dampers to improve the efficiency of the air circulation on both sides of and around the disc.